In recent years, as a method for clarifying river water to be used as tap water or the like, a filtration method using a multiporous hollow-fiber membrane that is advantageous in safety improvement of treated water and space-saving of facilities is now being widely employed. Three performances is required of a multiporous hollow-fiber membrane, that is, a high blocking performance capable of definitely removing bacterium such as Cryptosporidium and turbid components, a high permeability performance for treating a large amount of water, and high strength for enabling long-term use under a wide range of operation conditions including chemical cleaning and a high operating pressure.
If a multiporous hollow-fiber membrane is used in the field of treating water in particular, an external pressure filtration method capable of securing a large filtration area is principally employed. Therefore, a multiporous hollow-fiber membrane is required to have high strength against an external pressure direction, namely, high compression resistance strength, so that the hollow-fiber membrane cannot be crushed in a filtration operation.
Patent Literature 1 discloses a porous polyethylene hollow fiber and a process for producing the same by employing a stretching-opening method. On the other hand, a thermally induced phase separation method different from the stretching-opening method is known as a process for producing a multiporous membrane (see, for example, Non-patent Literatures 1 to 4).